Cytomegalovirus and Cardiovascular Disease: A Hypothetical Role for Viral G-Protein-Coupled Receptors in Hypertension

Abstract

Cytomegalovirus (CMV) is a member of the β-herpesviruses and is ubiquitous, infecting 50%–99% of the human population depending on ethnic and socioeconomic conditions. CMV establishes lifelong, latent infections in their host. Spontaneous reactivation of CMV is usually asymptomatic, but reactivation events in immunocompromised or immunosuppressed individuals can lead to severe morbidity and mortality. Moreover, herpesvirus infections have been associated with several cardiovascular and post-transplant diseases (stroke, atherosclerosis, post-transplant vasculopathy, and hypertension). Herpesviruses, including CMV, encode viral G-protein-coupled receptors (vGPCRs) that alter the host cell by hijacking signaling pathways that play important roles in the viral life cycle and these cardiovascular diseases. In this brief review, we discuss the pharmacology and signaling properties of these vGPCRs, and their contribution to hypertension. Overall, these vGPCRs can be considered attractive targets moving forward in the development of novel hypertensive therapies.

Graphical abstract

Graphical Abstract.

Upon reactivation of cytomegalovirus, chemokines bind to US28 to initiate RhoA/Rho kinase signaling in endothelium and smooth muscle leading to reduced dilation and constriction, respectively.

Cytomegalovirus (CMV) is a DNA virus that belongs to the herpes family of viruses [(β-herpesviruses)^1]. CMV is widely distributed in the population and CMV infection has been associated with several cardiovascular diseases including stroke, atherosclerosis, post-transplant vasculopathy, and hypertension.² Herpesviruses, including CMV encode viral G-protein-coupled receptors (vGPCRs) that alter the host cell by hijacking signaling pathways important to the viral life cycle and these cardiovascular disease conditions.³ In this brief review, we discuss the pharmacology and signaling properties of these vGPCRs, and their contribution to hypertension. Because of their prominent role, vGPCRs are promising drug targets for the treatment of hypertension. For those individuals desiring a more in-depth treatment of the subject several recent reviews are recommended.

FAMILY OF vGPCRs

Herpesvirus-encoded GPCRs

Human cytomegalovirus (HCMV) is a member of the β-herpesviruses and is ubiquitous, infecting 50%–99% of the human population depending on ethnic and socioeconomic conditions.³ It causes immediate, life-threatening diseases only in individuals with impaired, immature, or senescent immunity. However, the severity of HCMV on human health is understated since HCMV is associated with hypertension, and uncontrolled, elevated arterial pressure can lead to disability, poor quality of life or even a deadly heart attack or stroke.^3–7

HCMV has a large double-stranded DNA (dsDNA) genome that encodes over 200 proteins involved in control of viral gene expression, DNA replication (e.g., DNA polymerase), viral entry, cell-to-cell spread, immune evasion, and pathogenesis.² Among these regulatory proteins, HCMV encodes G-protein-coupled receptors (viral GPCRs). HCMV encodes 4 vGPCRs that alter the host cell by hijacking cellular pathways and play important roles in the viral life cycle: US28, US27, JL33, and UL78—the first 3 have similarities to chemokine receptor structure.

Other important information about vGPCRs²:

• Mouse and rat CMV encode homologs of UL33 and UL78, but not US28 and US27.

• vGPCRs can signal in a ligand-independent, constitutively active manner via several G proteins (Gα_q, Gα_i, Gα_s, and Gα_12/13).

• US28 is activated by chemokines (natural and chimeric).

• UL33, M33, and R33 activate PLCβ via coupling to G_q/11 subunits and via βγ subunits released from α_i/o proteins.

• HCMV productively infects multiple cell types including macrophages, smooth muscle cells, and endothelial cells.

Pharmacological properties of vGPCRs

Homology to chemokine receptors

As noted above, HCMV encodes 4 vGPCRs (US27, US28, UL33, and UL78). These viral receptors share homology with human chemokine receptors, and US28 has the highest homology [(35% with CX3CR1)^8]. Due to this homology with human chemokine receptors, chemokines can bind to vGPCRs. Also, as with human chemokine receptors, the N terminus of the vGPCRs is necessary for ligand binding.⁹

Chemokine scavengers or sinks

It is possible that some vGPCRs could act as “chemokine scavengers” or “chemokine sinks.”³ Respectively, scavenging would change chemokine activity in a negative way and a sink would act to store chemokines for recruitment to amplify a chemokine signal.

Constitutively active receptors

Unlike most mammalian GPCRs, some vGPCRs (US28 and UL33) can signal in a ligand-independent manner and thus they have constitutive activity.³ Additionally, vGPCRs can couple, in a promiscuous manner, to several G proteins (Gα_q, Gα_i, Gα_s, and Gα_12/13) and signal constitutively.

Heteromerization or cross-talk with chemokine receptors

Cellular signaling mechanisms of human GPCRs can be positively or negatively modulated by vGPCRs. This occurs by either direct means (heterodimerization) or by indirect mechanisms [(signaling cross-talk; sharing of downstream signaling proteins; or transcriptional regulation of the GPCRs, ligands, or signaling proteins)^3,10–14].

Downstream signaling of vGPCRs

Downstream signaling of vGPCRs (Figure 1) occurs via known pathways of mammalian GPCRs to include US28 activation of phospholipase C via Gα_i/0 leading to elevated inositol 1,4,5-trisphosphate (IP₃) and activation of diacylglycerol (DAG). IP₃ leads to increased calcium release from intracellular stores and DAG activates protein kinase C (PKC) which phosphorylates specific target proteins. US28 also activates ERK2 (extracellular signal-regulated kinase 2), RhoA/Rho-associated protein kinase (RhoA/ROCK), Src (tyrosine kinase), nuclear localization of cytoplasmic NFAT (nuclear factor of activated T cells), NF-κB (nuclear factor κB), and p38-MAPK (p38 mitogen-activated kinase). These signaling pathways are known to play roles in vascular contraction and smooth muscle cell migration.

Figure 1.

Downstream signaling of vGPCRs occurs via known pathways of mammalian GPCRs to include US28 activation of phospholipase C via Gα_i/0 leading to elevated IP₃ and activation of DAG. IP₃ leads to increased calcium release from intracellular stores and DAG activates PKC which phosphorylates specific target proteins. US28 also activates ERK2, RhoA/ROC), Src (tyrosine kinase), nuclear localization of cytoplasmic NFAT NF-κB, and p38-MAPK. These signaling pathways are known to play roles in vascular contraction and smooth muscle cell migration. Abbreviations: DAG, diacylglycerol; IP₃, 1,4,5-trisphosphate; PKC, protein kinase C; vGPCRs, viral G-protein-coupled receptors.

CARDIOVASCULAR DISEASE AND CMV

Hypertension and vascular dysfunction

CMV infection is associated with various chronic inflammatory diseases, including cardiovascular diseases.¹⁵ Hypertension is an important risk factor for morbidity and mortality caused by cardiovascular diseases, and many studies^5,6,16 have shown that patients with a CMV infection have an increased risk of hypertension. Furthermore, given that 31% of normotensive subjects and 53% of hypertensive subjects are infected with CMV, CMV may be a risk factor for but not necessarily an independent cause of hypertension. However, the association between CMV and hypertension remains unclear, and thus the effect of CMV infection on blood pressure is controversial.

In healthy individuals, both primary infection and the reactivation of latent virus rarely cause any significant clinical manifestations, owing to the robust immune response of the host. One of the mechanisms involved in the virus latency is dependent of BMPR2/YY1 (bone morphogenetic protein receptor type 2/yin yang 1) signaling axis in progenitor cells.¹⁷ Maintenance of viral latency requires a vigorous and vigilant immune system, highly dependent upon competent cytotoxic T cells, and any changes in immune status tend to promote viral reactivation.¹⁸

To clarify the association between CMV infection and hypertension, meta-analysis studies were conducted.^5,19 The largest one included a total of 11,878 participants to 9 trials, of whom 3,864 were patients with hypertension and 8,014 were normotensive controls. The 9 studies included were conducted in various geographical areas: 1 in Denmark,²⁰ 5 in China,^6,16,21,22 1 in Iran,²³ and 2 in United States.^7,24 All studies from China described an association between CMV and hypertension; however, the study from Denmark and one of the studies from United States did not reach the same conclusion.^20,24 The other study conducted in the United States found an association between CMV antibodies and hypertension only in women.⁷ The meta-analysis indicated that CMV is closely associated with human hypertension, but only in Chinese individuals.¹⁹

Infection with CMV as a cause of hypertension was discussed by Cheng et al. Using a mouse model infected with CMV for 10 weeks, this study demonstrated that infection was associated with an increase in systolic and diastolic blood pressure, independent of atherosclerotic plaque formation in the aorta. The authors suggested that the increase in blood pressure was mediated by inflammation and activation of the renin–angiotensin system. As evidence for this mechanism, infected mice exhibited increased serum concentration of IL-6 (interleukin-6), TNF-α (tumor necrosis factor-alpha), MCP-1 (monocyte-chemoattractant protein-1), and Ang II (angiotensin-II). Additionally, human and mouse renal cells and human endothelial cells infected with CMV displayed a concentration-dependent increase in renin expression.²⁵ The authors concluded that CMV infection is a risk factor for increased blood pressure.

An elegant study by Li et al. using a microarray-based miRNA expression profiling found high expression of hcmv-miR-UL112, which is a HCMV-encoded miRNA, in hypertensive patients. Interestingly, there were no differences in the miRNA expression levels between hypertensive patients receiving antihypertensive treatments and those that were not. In addition, seropositivity, log-transformed copies of HCMV, and hcmv-miR-UL112 were independently associated with an increased risk of hypertension. The authors also showed that IRF-1 (Interferon Regulatory Factor 1), which plays a role in inflammation and vascular disease pathogenesis, is a direct target of hcmv-miR-UL112.¹⁶

Vascular dysfunction is one of the main causes and complications of hypertension.²⁶ The origin of vascular dysfunction in hypertension has been studied for many decades and CMV infection is a potential contributing factor to this disorder.⁵ Grahame-Clarke et al. analyzed vascular function in patients with positive and negative HCMV serology using venous occlusion plethysmography and responses to different agonists. Individuals who were seropositive for HCMV had reduced responses to bradykinin and glyceryl trinitrate, suggesting that HCMV-seropositive individuals have endothelial dysfunction and impaired responses to NO (nitric oxide).²⁷ Moreover, a negative correlation between CMV DNA in plasma and endothelial function, independent of other cardiovascular risk factors, was found in patients with ST-elevation myocardial infarction.²⁸

In fact, CMV can infect endothelial cells and activate mechanisms that lead to inflammation and dysfunction. HCMV infection of endothelial cells leads to decreased expression of RGS5 (regulator of G-protein signaling 5) and promotes proliferation.²⁹ In addition, CMV can activate coagulation cascades and contribute to thrombus formation by increasing von Willebrand factor, ICAM-1 (intercellular adhesion molecule-1), and VCAM-1 (vascular cell adhesion molecule-1) in endothelial cells.³⁰ Hypertensive patients with positive CMV IgM antibodies presented high plasma levels of endothelin and P-selectin, suggesting that active CMV infection may aggravate vascular endothelium lesions in hypertension.²²

The release of inflammatory mediators by endothelial cells can be caused by nearby tissue inflammation or by direct infection of endothelial cells. Endothelial cells are activated by a wide range of stimuli, including shear stress, DAMPs (damage-associated molecular patterns), PAMPs (pathogen-associated molecular patterns), and inflammatory mediators.²⁶ van de Berg et al. proposed a model for noninfected endothelial cell damage during CMV latency. Local reactivation of CMV leads to maintenance of CD4- and CD8-specific effector T cells, which conduct to systemic immune and endothelial cells activation, leading to the production of chemokines and adhesion molecules.³¹ Pachnio et al. confirmed that CMV-specific CD4 T cells carry a chemokine receptor (CX3CR1) that directs the T cells to activate endothelial cells within blood vessels, suggesting that CMV immune response activation may lead to vascular dysfunction.³²

CMV infection also impairs the nitric oxide synthase (NOS)/NO signaling in human microvascular and aortic endothelial cells.³³ There are 3 isoforms of NOS: endothelial NOS (eNOS), neuronal NOS (nNOS), and inducible NOS (iNOS); the first 2 are constitutive enzymes that contribute to vascular homeostasis, and iNOS is activated mainly by stress signals/stimuli.³⁴ Infection of endothelial cells induces iNOS expression and leads to increased superoxide anion generation, reduced DDAH (dimethylarginine dimethylaminohydrolase) activity and thereby increased ADMA (asymmetric dimethylarginine) formation. ADMA is an endogenous inhibitor of eNOS, and CMV-induced accumulation of ADMA is linked to reduced activity of DDAH, the enzyme that degrades ADMA.³⁵ In addition, CMV also suppresses Akt and activates p38 MAPK causing eNOS inhibition and aortic endothelial cell dysfunction.

Despite the fact that endothelial cells can be infected by CMV and this viral infection can cause endothelial dysfunction, some studies have shown that endothelial cells do not contribute to HCMV dissemination.³⁶ A study showed that vascular endothelial and smooth muscle cells are unlikely to be important sites of CMV latency in vivo.³⁷ Nevertheless, pericytes, which are multipotent cells of the vascular system and direct interaction with endothelial cells, can be infected by CMV, and the virus can replicate within pericytes, thus contributing to vascular inflammation.³⁸

Although the contribution of CMV to hypertension is evident, the mechanisms that lead to viral infection, causing vascular dysfunction and increased blood pressure still need to be investigated.

Sympathetic activation

Stressful conditions induce hypothalamic–pituitary–adrenal and sympathetic–adrenal–medullary axes activation, leading to the release of several mediators, such as catecholamines and glucocorticoids. Epinephrine may induce molecular events that promote (re)activation of CMV expression and replication. Prosch et al. described that within a few days following the highly stressful event of acute myocardial infarction, all patients developed CMV antigenemia.³⁹ The authors observed a strong correlation between plasma catecholamine peak levels in the early postinfarction period and the frequency of CMV antigen-positive cells after 1 week.

Though acute responses to stress can be positive, long duration or chronically high levels of stress hormones negatively affect the regulation of the immune system and its individual components.⁴⁰ Changes, both in form (phenotype) and function (killing capacity), of many immune cells result in decreased cell-mediated immunity, which facilitates opportunistic reactivation of latent viruses.⁴¹ CMV is the only β-herpesvirus known to reactivate in astronauts and this finding coincided with high cortisol levels and immune system dysregulation.¹⁸ Hypothalamic–pituitary–adrenal and sympathetic–adrenal–medullary axes activation occurs during spaceflight, as indicated by increased levels of stress hormones, including cortisol, dehydroepiandrosterone, epinephrine, and norepinephrine. These changes, along with decreased cell-mediated immunity, contribute to the reactivation of latent herpes viruses in astronauts.

Evidence have also shown that CMV infection impairs arterial activity by activation of the sympathetic nervous system. In a rat model of irradiation-induced immunosuppression, CMV infection was found to alter heart rate regulation causing tachycardia, changes in arterial reactivity, and marked blood pressure reduction.⁴² The arterial changes were more marked in small arteries than in the aorta, which was hyporesponsive to adrenergic stimulus. These alterations could not be reversed by the NOS inhibitor l-NAME, but were prevented by the alpha-1 antagonist prazosin, demonstrating that excessive α₁ adrenergic stimulation of arterial smooth muscle contributes to the selective alteration of excitation–contraction coupling. This suggests that CMV infection results in vascular failure independently of NOS, but via activation of the sympathetic nervous system.⁴²

Vascular stiffness

Recent research has shown that CMV seropositivity is independently associated with increased arterial stiffness in patients with chronic kidney disease. The exact mechanisms by which CMV infection contributes to vascular stiffness remain unclear, but there is evidence of the contribution of certain immune cell subsets, particularly memory T cells,⁴³ Th1,⁴⁴ senescent CD8 T cells,⁴⁵ and inducible Tregs.⁴⁶

CD4 and CD8 effector memory T cells have a destabilizing effect on atherosclerotic plaques, mainly in older men.⁴³ CMV infection has a positive association with memory T cells subsets, that is associated with carotid-to-femoral pulse-wave velocity (PVW), a cardiovascular risk factor to atherosclerosis. Interestingly, this finding was described only in older white men and not in women.⁴⁷

Chanouzas et al. described that one of the host cellular immune responses to CMV, namely the cytotoxic CD4⁺CD28^null T cell, is a Th1-skewed proinflammatory T cell that produces IFN-γ (interferon-gamma) and TNF-α in response to stimulation with CMV lysate.⁴⁴ The expansion of this cell type is independently linked to PVW and arterial stiffness in patients with ANCA (anti-neutrophil cytoplasmic antibody)-associated vasculitis, an autoimmune disease.

The other cell type that contributes to CMV infection and vascular disease is senescence T cells. Senescence T cells produce a lot of proinflammatory cytokines, exerts greater cytotoxicity and is associated with inflammatory diseases, including cardiovascular disease.⁴⁸ In humans, CMV is known to be one of the most important antigens for repetitive T-cell stimulation inducing senescence.⁴⁹ Yu et al. demonstrated, in a Korean population, that PVW is strongly correlated with the frequency of CMV-specific senescent CD8 T cells. In this study, arterial stiffness was associated with CMV-specific IFN-γ and TNF-α secretion and with the cytotoxic degranulation of CD8+ T cells.⁴⁵

Terrazzini et al. also observed an association between inducible Tregs specific to CMV peptides and blood pressure.⁴⁶ This type of T cell is induced by the same antigens as those that induce a conventional T-cell response and is involved in attenuating the immune response. This finding was associated with old age, specifically in older women, and the authors suggested that it can contribute to the delay of significant cardiovascular events by driving a more chronic process.

Atherosclerosis

Acute CMV infection causes ascites, myocarditis, pulmonary artery inflammation, and infiltrate of macrophages and T lymphocytes accelerating inflammation in vascular tissue overexpressing MCP-1.⁵⁰ Some authors suggest that the presence of CMV in inflamed tissues implicates this virus in atherogenesis.⁵⁰ A meta-analysis involving 30 studies, 3,328 patients with atherosclerosis, and 2,090 controls showed that HCMV infection is significantly associated with an increased risk for atherosclerosis.⁵¹ This study indicated that CMV infection in Asian populations was high compared with other ethnicities.

A handful of reviews cover the potential role of CMV in atherosclerosis.^52–55 CMV has direct and indirect effects on atherosclerosis. Many studies have identified CMV antigens in resected atherosclerotic plaques.⁵⁴ Besides, the virus infection can contribute to the main pathways involved in atherogenesis, such as chronic inflammation, plaque development, plaque rupture, and vascular events.⁵³ CMV can lurk in many types of cells, like epithelial cells, endothelial cells, macrophages, and so on, causing immune status change and chronic inflammation.⁵⁶ The potential molecular mechanisms activated by CMV, detailed by Zhu et al., are generation of reactive oxygen species, dysregulation of lipid metabolism, miRNA regulation, ER (endoplasmic reticulum) stress, and inhibition of autophagy.⁵²

Vascular smooth muscle cells can also be infected by CMV with a pro-atherogenic effect. In vitro, the virus directly activates the PDGF (platelet-derived growth factor) system, which may promote atherogenesis and restenosis by activation of human coronary artery smooth muscle cell proliferation and neointima formation.⁵⁷ CMV infection can also change vascular smooth muscle cell function through ADAM9 expression, a metalloproteinase that is implicated in the development of vascular disease.⁵⁸

CMV infection increases reactive oxygen species in vascular smooth muscle cells, leading to excessive ox-LDL (oxidized-low-density lipoprotein) production. Furthermore, CMV infection of murine aortic smooth muscle cell produces plaques, fibroblast differentiation and induces calcification.

The effect of CMV infection on the mouse heart was described as muscle thickening, collagen accumulation in the epicardium, fibrosis, and increased heart rate. Interestingly, the use of the anti-CMV drug, ganciclovir, appears to lower heart transplant related atherosclerosis.⁵⁹

Stroke

Although CMV infection is associated with some cardiovascular diseases, conflicting results have been found when analyzing the effect of CMV infection on the risk of stroke. A study of 2,844 men and 3,257 women in the US National Health and Nutrition Examination Survey found a strong association between CMV and stroke only in women.⁶⁰ Conversely, a study of 8,531 individuals in the United Kingdom Biobank found no association between CMV seropositivity and risk of CVD, ischemic heart disease, or stroke.⁶¹

In a meta-analysis on the effect of CMV infection on stroke risk comprising 3 studies,² 2 reported that CMV infection was associated with slightly or moderately increased relative risk of stroke,^62,63 and 1 that CMV infection was associated with slightly decreased risk of stroke.⁶⁴ The result of the meta-analysis estimated an increased relative risk of stroke of 1.16, and this result was nonsignificant. However, in this same meta-analysis, the authors found strong indications that CMV infection may predispose to CVD events.

CHEMOKINES AND VIRAL GPCRs COLLIDE TO EXPLAIN HCMV-RELATED HYPERTENSION

Chemokines

Chemokines are small, highly conserved families of secreted proteins composed of cytokines or signaling proteins,⁶⁵ responsible for regulating chemotaxis.⁶⁶ Chemokines perform their function by associating with over 20 chemokine receptors which are GPCRs,⁶⁷ thereby affecting a variety of biological processes such as implantation of the early conceptus, the migration of cells during embryonic development, wound healing, and maintenance of innate and adaptive immunity. When the physiological role of chemokines is subverted or chronically amplified, disease often follows, such as cancer,⁶⁸ cardiovascular diseases,⁶⁹ liver diseases,⁷⁰ and intestinal diseases.⁷¹ Chemokines are classified into 4 major families, CXC, CC, XC, and CX3C, by the arrangement of their N-terminal cysteines.⁶⁷ CXC and CC are the major subfamilies of chemokines. Among the CC chemokine subfamilies, CCL2, also known as MCP-1, was the first to be discovered.^72,73 It consists of 76 amino acids at 13 kDa and has 2 adjacent amino-terminal cysteine residues.⁷⁴ CCL2 is primarily secreted by immune cells and its expression can be either persistent or inducible. In addition, smooth muscle cells, endothelial cells, thylakoid cells, and fibroblasts are also capable of producing CCL2.^75–77 Many mediators can induce CCL2 expression, including IL-1, IL-4, IL-6, TNF-α, TGF-β (transforming growth factor-beta), and IFN-γ.^78,79 CCL2 regulates the migration and infiltration of monocytes, macrophages, memory T lymphocytes, and NK (natural killer) cells.^66,74 CCL2 binds to a large number of receptors to coordinate inflammatory monocyte transport among bone marrow, circulating and atherosclerotic plaques.⁶⁹ CCL2 binding to CCR4 activates myosin light chain phosphorylation and regulates cell motility and metastasis of tumors.⁸⁰ Additionally, CCL2 can bind to atypical chemokine receptors (ACKRs), including ACKR1 and ACKR2, which can alter chemokine gradients.⁸¹ Although CCL2 can bind to a variety of receptors, CCR2 is still considered to be the primary receptor for the chemokine.

CCL2–CCR2 axis and hypertension

In 2017, the American Heart Association along with the American College of Cardiology established new guidelines for blood pressure management and defined hypertension as a blood pressure at or above 130/80 mm Hg. Hypertension contributes to a variety of serious health complications including cardiac failure, stroke, and renal failure. Chronic inflammation in the vascular wall contributes to the initiation and maintenance of elevated arterial pressure in patients and animal models of the condition.⁸² The CCL2/CCR2 axis is a necessary mediator of the vascular inflammatory response and has been shown to play a critical role in vascular remodeling, and hypertrophy via monocyte infiltration and macrophage recruitment.^83,84 Angiotensin II stimulates the vascular expression of the CCL2 gene via the Ang II type-1 (AT1) receptor.⁸⁵ CCL2 expression is elevated in vascular tissue of animals made hypertensive by Ang II treatment.⁸⁶ CCR2 played a crucial role in macrophage infiltration, vascular hypertrophy, inflammation, and remodeling in animal models of Ang II-induced hypertension. In CCR2-deficient mice, infiltration of arterial wall macrophages was nearly abolished and vascular hypertrophy was significantly reduced.⁸³ Ang II-induced inflammation and remodeling of blood vessels was significantly attenuated in CCR2 knockout mice.⁸⁴ In 740 hypertensive patients, soluble CCL2 was observed to be increased and this correlated with the extent of organ injury.⁸⁷ Further, CCR2 expression was elevated in monocytes of hypertensive patients and decreased after treatment with Ang II receptor blockers.⁸⁴

The CCL2–CCR2 axis plays an important role in the progression of hypertensive kidney damage. In patients with salt-sensitive hypertension, the CCL2–CCR2 signaling pathway is enhanced and exacerbates renal injury.⁸⁸ Lack of CCL2 reduced cortical atrophy and reduced the number of infiltrating monocytes, thereby preventing chronic renal injury in mice with renal hypertension.⁸⁹ The CCR2 antagonist RS102895 prevented leukocyte infiltration into the kidney and attenuated salt-sensitive hypertension and renal injury. Thus, the CCL2–CCR2 axis leads to salt-sensitive hypertension by altering renal leukocyte infiltration.⁹⁰

HYPOTHESIS

As noted above, CMV is usually latent in most individuals without symptomology. During reactivation of CMV, macrophages, endothelial and vascular smooth cells can become infected, and the virus causes these cells to express vGPCRs to reinitiate the viral life cycle (Figure 2). Stimuli^39,82,91,92 that reactivate the virus are: psychological stress (sympathetic hyperactivation), inflammatory cytokines (IL-6, TNF-α); ischemia reperfusion injury; DNA damage; and oxidative stress. Once activated, these vGPCRs hijack signaling pathways that are also important for vascular function (remodeling and constriction). In humans, the vGPCRs, US28 and UL33 would be the major players in altering vascular function. Chemokines bind to US28 to initiate RhoA/Rho kinase signaling in endothelium and smooth muscle leading to reduced dilation and constriction, respectively. Importantly, it is known that chemokines and their receptors play an important role in hypertension^82,93 and chemokine antagonists lower blood pressure.⁹⁴ Additionally, constitutive activity of UL33 would increase in vascular smooth muscle leading to stimulation of PLC and NF-κB to cause vasoconstriction and vascular remodeling, respectively (Figure 3). These changes in the vasculature would contribute to increased vascular resistance maintaining blood pressure at high levels in hypertension.

Figure 2.

During reactivation of CMV, endothelial and vascular smooth cells can become infected, and the virus causes these cells to express vGPCRs to reinitiate the viral life cycle. Stimuli that reactivate the virus are: psychological stress, inflammatory cytokines (IL-6, TNF-α); ischemia reperfusion injury; DNA damage; and oxidative stress. Once activated, these vGPCRs hijack signaling pathways that are also important for vascular function (remodeling, dilation, and constriction). In humans, the vGPCRs, US28 and UL33 would be the major players in altering vascular function. Chemokines bind to US28 to initiate RhoA/Rho kinase signaling in endothelium and smooth muscle leading to reduced dilation and constriction, respectively. Abbreviations: CMV, cytomegalovirus; vGPCRs, viral G-protein-coupled receptors.

Figure 3.

Upon reactivation of cytomegalovirus, constitutive activity of UL33 is increased in vascular smooth muscle leading to stimulation of PLC and NF-κB to cause vasoconstriction and vascular remodeling, respectively.

SIGNIFICANCE.

G-protein-coupled receptors (GPCRs) constitute the largest family of membrane receptors with great diversity.⁶ Due to their role as important regulators of numerous cellular processes, GPCRs exhibit central relevance to the current clinical practice of medicine.

The appearance of viral vGPCRs in genomes of herpesviruses is probably a result of viral hijacking during coevolution with the respective hosts. Importantly, recent work⁷ suggests that SARS-CoV-2 may also potentially alter second messenger signaling cascades via activation of GPCRs. Thus, this work addresses a highly significant area of research and provides important strategies for the pharmacological treatment of hypertension associated with many ubiquitous viral infections.

FUNDING

R.C.W.: RO1 awards from the National Institute of Health - USA (DK132948 and HL134604), a PO1 award (HL13604), and a pilot award from the NIDDK Diabetic Complication Consortium; G.F.B.: Conselho Nacional de Desenvolvimento Científico e Tecnológico - Brazil (402664/2022-1).

DISCLOSURE

The authors have no conflicts of interest to declare. All coauthors have seen and agree with the contents of the manuscript and there is no financial interest to report.